A Missouri grand jury heard evidence for months before deciding not to indict Ferguson police Officer Darren Wilson in the Aug. 9 fatal shooting of Michael Brown. Some answers to common questions about the grand jury:

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Q: How was the grand jury different from other juries?

A: The grand jury could determine only whether probable cause exists to indict Wilson, not whether he is guilty. If the jury had returned an indictment, a separate trial jury would have been seated to decide whether to convict or acquit him.

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Q: How many people were on the grand jury and how were they selected?

A: The grand jury was composed of 12 people "selected at random from a fair cross-section of the citizens," according to Missouri law. The jurors, whose identities were kept secret, were 75 percent white: six white men, three white women, two black women and one black man. St. Louis County overall is 70 percent white, but about two-thirds of Ferguson's residents are black. Brown was black. The officer is white.

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Q: Was the grand jury appointed for this specific case?

A: No. It was appointed for a four-month term. The grand jury had been hearing routine cases around the time Brown was killed and then turned its attention to the shooting.

The jury's term was due to expire Sept. 10. That same day, county Judge Carolyn Whittington extended the term to Jan. 7 — the longest extension allowable by state law. The investigation was always expected to go longer than the typical grand jury term.

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Q: How often did the grand jurors meet?

A: Their normal schedule was to meet once a week. In this case, they met on 25 separate days, St. Louis County Prosecuting Attorney Bob McCulloch said.

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Q: Who was inside the grand jury room?

A: The jury, a prosecutor and a witness. Grand jury proceedings are closed to the public.

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Q: What happened when the grand jury convened?

A: Prosecutors presented evidence and summoned witnesses to testify. A grand jury is a powerful tool for investigating crimes because witnesses must testify unless they invoke the Fifth Amendment of the U.S. Constitution, which protects against self-incrimination.

Typically, grand jurors hear a condensed version of the evidence that might be presented at a trial. In the Ferguson case, grand jurors received more extensive evidence and testimony. They heard 70 hours of testimony, McCulloch said.

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Q: Who testified to the grand jury?

A: McCulloch said the grand jury heard from 60 witnesses, including Wilson and three medical examiners. It wasn't clear how many appeared in person or submitted testimony in another manner, such as through a statement.

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Q: What specific charges could have been filed?

A: At the lower end was second-degree involuntary manslaughter, which is defined as acting with criminal negligence to cause a death. It is punishable by up to four years in prison.

First-degree involuntary manslaughter, defined as recklessly causing a death, is punishable by up to seven years in prison. Voluntary manslaughter, defined as causing a death "under the influence of sudden passion arising from adequate cause," is punishable by five to 15 years in prison. Second-degree murder is defined as knowingly causing a death, or acting with the purpose of causing serious physical injury that ends up resulting in death. It is punishable by life in prison or a range of 10 to 30 years.

The most serious charge, first-degree murder, can be used only when someone knowingly causes a death after deliberation and is punishable by either life in prison or lethal injection.

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Q: Did charges require a unanimous vote?

A: No. Consent from nine jurors is enough to file a charge in Missouri. The jury could also choose not to file any charges. MCulloch

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Q: Can jurors speak to the public?

A: No. Disclosing evidence, the name of a witness or an indictment can lead to a misdemeanor charge.

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Q: What is being publicly disclosed now that grand jurors have reached a decision?

A: If Wilson had been charged, the indictment would have been made public, but the evidence will be kept secret for use at a trial. If Wilson is not indicted, St. Louis County prosecutor Bob McCulloch has said he was taking the unusual step of releasing transcripts and audio recordings of the grand jury investigation.

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Q: What preparations have been made?

A: Missouri Gov. Jay Nixon has declared a state of emergency and activated the National Guard to help state and local police in case of civil unrest. Some school districts called off classes for Monday and Tuesday. Police have undergone training pertaining to protesters' constitutional rights and have purchased more equipment, such as shields, helmets, smoke canisters and rubber bullets.